1. Field of the Invention
The present invention relates to lithium aluminate which is particularly useful for an electrolyte-retainer plate in a Molten Carbonate Fuel Cell (MCFC) and to an industrial preparation process thereof.
2. Description of the Related Art
An electrolyte-retainer plate of MCFC is required to have such characteristics as high molten carbonate retaining property, alkali resistance and heat resistance since it is used for retaining a mixed molten carbonate such as Li.sub.2 CO.sub.3 and K.sub.3 CO.sub.3 in an elevated temperature region around 650.degree. C. As a material which satisfies the required characteristics, presently lithium aluminate has been used as a structural material of the electrolyte-retainer plate, particularly finely divided .gamma.-type lithium aluminate having a relatively large specific surface area and excellent electrolyte retaining property has been used.
As for the technologies of producing lithium aluminate having such a large specific surface area, many suggestions have been made including those disclosed in Japanese Patent Laid-Open Nos.60-65719, 60-151975, 61-295227, 61-295228, 63-270311, 1-252522, and 2-80319. The main points of these known methods lie in increasing the specific surface area by controlling the denseness of the structure through burning a mixture of alumina and lithium hydroxide or lithium carbonate at a temperature in a range of from 600 to 1000.degree. C., or by carrying out secondary treatment such as hydration treatment or a treatment to impart porosity.
In the specification of Japanese Patent Publication No.6-37292, a process for preparing porous lithium aluminate coarse particles has been disclosed in which silica particles and alumina particles are bonded to prepare coarse particles then they are heated to 500-1000.degree. C. in a carbonate containing lithium ion so that the silica particles in the coarse particles are eluted into the carbonate to form pores and the alumina having pores is combined with lithium to form lithium aluminate.
However, when the .gamma.-type lithium aluminate produced by the above-mentioned conventional technique is exposed to a high temperature in a molten electrolyte for a long time, such phenomenon appears that the .gamma.-type structure is partly transformed into .alpha.-type, or the specific surface area is decreased by grain growth. Accordingly, when it is formed into an electrolyte-retainer plate for MCFC, it has a defect that the electrolyte retention capability may drop suddenly during its use to deteriorate the battery life.
The process of preparing porous lithium aluminate disclosed in the above-mentioned Japanese Patent Publication No.6-37292 has been proposed as a technique to alleviate such defect, however, it has problems such as being a long and complicated process since it requires special process to carry out ion exchange of lithium after obtaining coarse particles of silica-alumina, and difficulty in carrying out ion exchange completely.
As can be seen from the above description, the requirements for improving the long-life of MCFC which are becoming more and more severe, including high level of retention capability for the molten carbonate, high alkali resistance and high heat resistance, cannot be sufficiently satisfied with the conventional .gamma.-type lithium aluminate preparation technique, besides, the conventional techniques still have some problems to be cleared before they can be used as industrial production means.
Accordingly, an object of the present invention is to solve the above-mentioned conventional problems, and in i particular, to provide .gamma.-type lithium aluminate having excellent heat stability and chemical stability in a molten carbonate when used for an electrolyte-retainer plate of MCFC, and an industrial preparation process thereof.